The overall goal of this ACE Center is to develop a translational program of research, from gene to behavior, around a clinically important, tractable, and understudied clinical problem in autism: the Insistence on Sameness (IS). The aim of Project 2 is to seek a mechanistic understanding of behavioral features of IS in cognitive neuroscience and biochemical terms. These levels of analysis are important in a translational Center. Ultimately, genes studied in Projects 1 &4 exert their clinical effect via specific alterations of cognitive, affective and biochemical brain systems. Precise measurement of these intermediate effects, or endophenotypes, can speed gene discovery and the understanding of how disease-related gene variants are relevant to the disorder. Similarly, efforts to develop new and better treatments for the clinic, the ultimate aim of Project 3, will have the greatest opportunity for success if guided by a better understanding of the targets for behavioral and psychopharmacological intervention. From a cognitive perspective, IS can be considered as a difficulty in set switching from highly prepotent response tendencies. From an affective perspective, a difficulty modulating distress can potentially precipitate IS, disrupt the ability to shift set from IS, and be a dramatic result of efforts to terminate IS behaviors. Cognitive and affective impairments related to IS may thus be reciprocal, and both are amenable to study in the clinic and animal models. At the biochemical level, psychopharmacology and animal model research suggest that 5HT disturbances may contribute to IS. For these reasons, we focused our research program on evaluating behavioral flexibility, the modulation of affective responses, set shifting under affective stress conditions, and the ability to overcome strong prepotent response tendencies across our translational research program. Clinical studies will use neurobehavioral testing and fMRI to test hypotheses about the relationship between disturbances in these domains and IS, and about how atypical organization of frontostriatal and frontolimbic circuitry may contribute to the manifestation of IS. Rodent studies of acute SSRI and 5HT6 effects will provide a mechanistic understanding of how activation states of 5HT receptor systems impact set switching behavior under different circumstances. Rodent studies will be complemented by and inform interpretation of findings from a treatment study of High IS patients with the SSRI escitalopram using neurobehavioral and fMRI outcomes. Thus, the aim of Project 2 is to develop an integrated, mechanistic understanding of IS from the behavioral to the neurophysiological to the biochemical level.